It is known to use vacuum and the properties of vacuum in numerous sectors of industry, particularly in the medical field or the food field.
Thus, in the medical field, hospital buildings are often traversed by a network of vacuum conduits permitting bringing a source of underpressure, which is to say vacuum, to the hospital rooms or the operating rooms at which said source of underpressure or vacuum can be used for various and sundry ends, for example to be used in the aspiration of body fluids by means of suitable instruments, for example during a surgical procedure or the like.
However, the problem which is conventionally faced is that of being able to control the level of underpressure, which is to say vacuum, prevailing in the network of vacuum conduits, so as to be able to ensure, at the user sites, a pressure level sufficiently low to be able to ensure the various applications to which the vacuum is destined, in general a level of underpressure or vacuum less than 0.5.times.10.sup.5 Pa.
To solve this problem, several installations and processes for regulation have already been proposed.
Thus, there is known an installation or central unit for a medical vacuum permitting maintaining a level of underpressure less than 0.5.times.10.sup.5 Pa in one or several vacuum conduits, comprising one or several vacuum pumps connected pneumatically to said vacuum conduits, means for determining the pressure in said vacuum conduits and transmitting information as to pressure to pilot means controlling the operation or the stopping of the vacuum pump or pumps as a function of the pressure level detected by said means for determining pressure.
However, this type of device, although simple in its design, has the drawback of requiring moreover the use of several storage capacities of large volume connected to the vacuum conduits, so as to be able to respond substantially instantaneously to high demand at the user sites for vacuum and/or to avoid rapid deterioration of the vacuum pumps.
Thus, the operation of the installation sites requires fairly abrupt and incessant starting of the vacuum pumps to permit adjusting the level of underpressure, which is to say the vacuum level, in the vacuum conduits between a maximum pressure value and a minimum desired pressure value, said frequent starts of these pumps carrying the risk of rapidly damaging the motors driving said vacuum pumps.
Because of this, to avoid this problem, it is necessary to add several buffer capacities of large volume, so as to minimize the frequency of starting said pumps; the network of vacuum conduits having a too small buffer capacity to be able to perform by itself this role.
It is easy to understand that the fact of having to provide several buffer capacities of large volume gives rise to drawbacks, namely large size due to the size of these buffer capacities, which gives rise to difficulty of transporting these capacities to user sites and difficulty of installing these buffer capacities which require a large space, in general an area especially adapted to receive them.
So as to solve the problems existing with this type of installation, it has been proposed to add electrovalves at the outlet of each vacuum pump.
Because of this, each vacuum pump operates during a predetermined time, for example 6 to 10 minutes, during which the electrovalve is in the open position, which permits being able to decrease the pressure to the desired value of underpressure.
Then, when the desired value of underpressure is reached, the electrovalve closes whilst the vacuum pump continues to operate for the predetermined period, for example 6 to 10 minutes, after which the pump stops.
When the pressure level again reaches a maximum threshold of underpressure, the electrovalve opens again and the preceding steps are repeated.
However, it has been noted that the use of electrovalves between each vacuum pump and the conduit to which is connected each vacuum pump, gives rise to large problems of pressure drop which cause a large decrease of output of each of said vacuum pumps, which can amount to 10% or even more.
Moreover, the electrovalves that exist at present are subject to problems of reliability, in particular in the case of membrane electrovalves, because these latter are not especially designed for pressures below 10.sup.5 Pa, which is to say for vacuum.
Although it would be possible to replace membrane electrovalves by electrovalves better adapted to vacuum, such as so-called "slide valve" electrovalves, these latter are, on the one hand, of a fairly high cost, and on the other hand, do not permit solving the problem of operation for vacuum pumps, when the electrovalve is closed and when, on the other hand, the predetermined operating time of the vacuum pump has not totally elapsed.